Evolution of Databases from Centralized to Distributed

There are five major developmental trends that affect the current nature of distributed databases.
While their time frames have overlapped, each has been the natural successor to the one it succeeded and improved in some manner over the earlier technology. They are:

1. 1960s: Monolithic databases
2. 1970s: Distributed databases
3. 1980s: Personal databases
4. 1980s: Networks of databases
5. 1990s: Networked networks of databases

For years, serious business databases were monolithic systems^[1] running only on one large machine, accessed by dumb terminals. In the late 1980s, databases evolved so that an application, called a client, on one machine could run against a database, called a server on another machine. At first, this client/server database architecture was only used on mini and mainframe computers.
Between 1987 and 1988, vendors like Oracle Corporation first moved the client function and then the database server down to microcomputers and Local Area Networks (LANs).
Today, the client/server concept has evolved to cover a range of approaches to distributing the processing of an application in a variety of ways between different machines. An example of the client-server approach is the SQL-server system, from Microsoft. The SQL-Server system is run on a server machine, which is usually a fairly powerful PC. A client program is run, usually on a separate machine, and makes requests for data to SQLserver via a local area network (LAN). The application program would typically be written in a language such as C# or Java. This approach allows multiple client machines on the network to request the same records from the database on the server. SQLserver will ensure that only one user at a time modifies any specific record.

When an organization is geographically dispersed, it may choose to store its databases on a central database server or to distribute them to local servers (or a combination of both). It is important to note the differences between distributed databases and centralized databases. In a centralized database, the database software maintains tight control over the data in terms of both security and access. The system can be backed up and recovered as a single unit, and most important, the business relationships between the data items are controlled and maintained within one environment. Centralized databases have been criticized for having a single point of failure. It is argued that a centralized data repository is at the mercy of the single processor.

On the other hand, a failure of a single processor in a distributed network will leave the remaining database nodes intact and running. This is a compelling reason to use Oracle Network Services to isolate and distribute business databases on separate processors.
A distributed database is a single logical database that is spread physically across computers in multiple locations that are connected by a data communications network. We emphasize that a distributed database is truly a database, not a loose collection of files.
The distributed database is still centrally administered as a corporate resource while providing local flexibility and customization. The network must allow the users to share the data; thus a user (or program) at location A must be able to access (and perhaps update) data at location B. The sites of a distributed system may be spread over a large area (for example, the United States or the world) or over a small area ( a building or campus). The computers may range from PCs to large-scale servers or even supercomputers. The final frontier of networks of networked computers is another evolution of the personal computer revolution. As personal computers became ubiquitous, LAN managers were created to link them together. With gateways to the midrange and mainframe systems, the desktop workstation has become merely a vehicle for exploring cyberspace. With tens of thousand of participating computers, and new nodes being added at a rate of 200 each day, the Internet is the ultimate in distributed databases. To address this trend, Oracle has introduced Oracle Network Services. In the next lesson, the changing attitudes toward distributed data will be discussed.

Before moving on to the next lesson, click the Quiz link below to test your knowledge of the evolution of Oracle networking.
Oracle Network - Quiz

[1]monolithic system: A software system is called "monolithic" if it has a monolithic architecture, in which differentiating features (data input and output, error handling, database access) are all interconnected, instead of containing architecturally separate components.